1. Field of the Invention
The present invention relates to a method of and an apparatus for establishing highlight and shadow densities used for gradation control of image data obtained through reading an original.
2. Description of Background Art
In an image processing apparatus such as a process scanner or the like, an image data obtained through reading an original is subjected to gradation control, whereby a recorded image having a desired gradation expression is obtained. In the gradation control, a highlight density and a shadow density must to be previously set in the image processing apparatus in order to define both ends of that density range of the original which is faithfully reproduced with the recorded image. The highlight and shadow densities are respectively measured with a densitometer at a highlight area and a shadow area which are selected by an operator as the brightest area and the darkest area in the original. The measured highlight and shadow densities are set in the image processing apparatus to be used for the gradation control.
Because the highlight and shadow densities have been manually measured and set in the image processing apparatus by an operator, its operation takes much time and skill. Furthermore, a densitometer is necessary for the measurement besides the image processing apparatus. If the highlight area and the shadow area are not large enough for an aperture of the densitometer to be set in those areas, which aperture is usually a circle of a few millimeters in diameter, the highlight and shadow densities cannot be set at appropriate values.
In order to overcome those problems, there have been proposed some techniques for establishing highlight and shadow densities properly and easily.
(a) Japanese Patent Laying Open Gazette 1984-15939
According to the technique disclosed in this gazette, a highlight point and a shadow point where the highlight and shadow densities are to be measured are first specified in an original by an operator, respectively. Then, brightnesses of the respective highlight and shadow points are automatically read with a photoelectric converter device in a graphic arts scanner, to thereby be converted to the highlight and shadow densities in the process scanner. In order to prevent measurement error in the highlight and shadow densities because of dust or the like on the original, the brightnesses of the highlight and shadow points are determined as average values of brightnesses in the vicinity of the highlight and shadow points, respectively. The highlight and shadow densities are automatically determined on the basis of the average values. As a result, measurement error due to dust or the like is reduced and the highlight and shadow densities are obtained with considerable accuracy.
Although appropriate highlight and shadow densities are established by this technique, manual operation is still necessary for specifying the highlight and shadow points.
(b) Japanese Patent Laying Open Gazette 1981-87044
(c) Japanese Patent Laying Open Gazette 1986-20042
The applicant of the present invention is also the applicant of these gazettes. According to the technique disclosed in these gazettes, sizes of candidate areas for the highlight and shadow points are measured first. If the candidate areas have enough size for measuring accurate densities, they are selected as the highlight and shadow points, respectively. Then, densities of the candidate areas are determined to be highlight and shadow densities, respectively.
However, an apparatus for measuring only the sizes of the candidate areas is necessary in the above techniques. Further, it is difficult to set highlight and shadow densities appropriately for an original lacking candidate areas of sufficient size for measuring accurate densities.
As described above, techniques in the art are insufficient for establishing proper highlight and shadow densities easily.
There is another issue concerning highlight and shadow densities for an original where average density is shifted from a normal value. Highlight and shadow densities for such an abnormal original need to be set shifted from a normal value according to magnitude of the shift of the average density in order to reproduce a relatively normal image. If an area of low density does not exist in an original or if the area has only an extremely small size, highlight density should be set at a lower value than usual in order for a recorded image to be reproduced normally. On the other hand, if an area of high density does not exist or if the area has only an extremely small size, shadow density should be set at a higher value than usual in order for a recorded image to be reproduced normally.
When an original image is abnormal where average density is shifted from a normal value, operation for establishing proper highlight and shadow densities depends on a skilled operator. Therefore, a method of and an apparatus for easily establishing proper highlight and shadow densities for an abnormal image has been an issue for years.